refactor(client): carve outward interfaces for the session, workspace, layout, slash, and conversation services

Feature packages now reach these domains through interface types only:
ISession/SessionFace (identity + prompt/cancel/loadOlder + the useSession
snapshot source), ISessions, IWorkspaces, ILayout, IConversation, and the
existing SlashServiceContract now actually mounted on Context.slash. The
concrete services implement their face; wire-pump and assembly entry
points stay on the classes. The provide-channel materialization and
current-projection logic moves into SessionProvideChannel so the
production service and the client test runtime share one implementation.
The workspaces service consumes sessions through the narrow SessionsPort.

packages/client/ui-layout/src/client/index.ts

@@ -17,14 +17,16 @@ import { ThemePresenter } from './theme-presenter.ts'
 
 // Contract surface only (export-convergence rule: cross-package consumers
 // keep a symbol exported; test-only/package-internal symbols live off /src).
-// LayoutService: the ctx.layout service class (consumers type against it).
+// ILayout: the ctx.layout face consumers and test fakes type against.
 // OwnerShare contracts below are the render-side halves registrants compose
 // against; the frame components and the store factory are package-internal.
 export { LayoutService } from './service.ts'
+export type { ILayout } from './service.ts'
 
 declare module 'cordis' {
   interface Context {
-    layout: LayoutService
+    /** The outward face only; the concrete service stays inside this plugin. */
+    layout: import('./service.ts').ILayout
   }
 }
 

packages/client/ui-layout/src/client/service.ts

@@ -14,8 +14,23 @@ import type { createLayoutStore } from './stores.ts'
 /** The layout store's bound action set (framework-baked, draft params peeled). */
 export type PanelActions = BoundActions<ReturnType<typeof createLayoutStore>>
 
+/**
+ * The outward layout face (`ctx.layout`): the panel transitions other
+ * plugins may trigger — and exactly what a test fake must supply. The
+ * attachPanels wiring hook stays on the concrete class (root-entry assembly
+ * only).
+ */
+export interface ILayout {
+  /** Toggle the sidebar panel (closed ⟷ contract default width). */
+  toggleSidebar(): void
+  /** Open the details panel (no-op when already open). */
+  openDetails(): void
+  /** Close the details panel. */
+  closeDetails(): void
+}
+
 /** Cross-plugin panel-action face (ctx.layout). */
-export class LayoutService {
+export class LayoutService implements ILayout {
   #panels: PanelActions | undefined
 
   /**